U.S. patent number 8,368,831 [Application Number 12/738,677] was granted by the patent office on 2013-02-05 for oligosiloxane modified liquid crystal formulations and devices using same.
This patent grant is currently assigned to Cambridge Enterprise Ltd., Dow Corning Corporation. The grantee listed for this patent is Terry Victor Clapp, Harry James Coles, William Alden Crossland, Anthony Bernard Davey, Omar Farooq, Martin Grasmann, Oliver Hadeler, Jonathan Paul Hannington, Russell Keith King, Fumito Nishida, Mykhaylo Pivnenko, Huan Xu. Invention is credited to Terry Victor Clapp, Harry James Coles, William Alden Crossland, Anthony Bernard Davey, Omar Farooq, Martin Grasmann, Oliver Hadeler, Jonathan Paul Hannington, Russell Keith King, Fumito Nishida, Mykhaylo Pivnenko, Huan Xu.
United States Patent |
8,368,831 |
Hannington , et al. |
February 5, 2013 |
Oligosiloxane modified liquid crystal formulations and devices
using same
Abstract
A liquid crystal formulation is described. The liquid crystal
formulation comprises a first oligosiloxane-modified nano-phase
segregating liquid crystalline material; and at least one
additional material selected from a second oligosiloxane-modified
nano-phase segregating liquid crystalline material, non-liquid
crystalline oligosiloxane-modified materials, organic liquid
crystalline materials, or organic non-liquid crystalline materials,
wherein the liquid crystal formulation is nano-phase segregated in
the SmC* phase, has an I.fwdarw.SmC* phase transition, with a SmC*
temperature range from about 15.degree. C. to about 35.degree. C.,
has a tilt angle of about 22.5.degree..+-.6.degree. or about
45.degree..+-.6.degree., and has a spontaneous polarization of less
than about 50 nC/cm2, and a rotational viscosity of less than about
600 cP. Devices containing liquid crystal formulations are also
described. The device has a stable bookshelf geometry, bistable
switching, and isothermal electric field alignment, a response time
of less than 500 .mu.s when switched between two stable states, and
an electric drive field of less than about 30 V/.mu.m.
Inventors: |
Hannington; Jonathan Paul
(Midland, MI), Clapp; Terry Victor (Bishop's Stortford,
GB), Nishida; Fumito (Midland, MI), King; Russell
Keith (Midland, MI), Farooq; Omar (Saginaw, MI),
Grasmann; Martin (Midland, MI), Crossland; William Alden
(Harlow, GB), Coles; Harry James (Sutton Ely,
GB), Davey; Anthony Bernard (Cambridge,
GB), Xu; Huan (Cambridge, GB), Hadeler;
Oliver (Cambridge, GB), Pivnenko; Mykhaylo
(Cambridge, GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
Hannington; Jonathan Paul
Clapp; Terry Victor
Nishida; Fumito
King; Russell Keith
Farooq; Omar
Grasmann; Martin
Crossland; William Alden
Coles; Harry James
Davey; Anthony Bernard
Xu; Huan
Hadeler; Oliver
Pivnenko; Mykhaylo |
Midland
Bishop's Stortford
Midland
Midland
Saginaw
Midland
Harlow
Sutton Ely
Cambridge
Cambridge
Cambridge
Cambridge |
MI
N/A
MI
MI
MI
MI
N/A
N/A
N/A
N/A
N/A
N/A |
US
GB
US
US
US
US
GB
GB
GB
GB
GB
GB |
|
|
Assignee: |
Dow Corning Corporation
(Midland, MI)
Cambridge Enterprise Ltd. (Cambridge, GB)
|
Family
ID: |
39204974 |
Appl.
No.: |
12/738,677 |
Filed: |
October 19, 2007 |
PCT
Filed: |
October 19, 2007 |
PCT No.: |
PCT/US2007/081940 |
371(c)(1),(2),(4) Date: |
June 28, 2010 |
PCT
Pub. No.: |
WO2009/051598 |
PCT
Pub. Date: |
April 23, 2009 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20100283927 A1 |
Nov 11, 2010 |
|
Current U.S.
Class: |
349/41 |
Current CPC
Class: |
C09K
19/406 (20130101); C09K 2323/023 (20200801); Y10T
428/1014 (20150115) |
Current International
Class: |
G02F
1/136 (20060101) |
References Cited
[Referenced By]
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WO |
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Other References
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Organosiloxanes With Unusual Electro-Optic Properties", 1993, pp.
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.
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.
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Molecular Tilt Approaching 45 Degrees", Feb. 2005, pp. 173-181;
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applicant.
|
Primary Examiner: Pak; Sung
Attorney, Agent or Firm: Brinks Hofer Gilson & Lione
Claims
The invention claimed is:
1. A liquid crystal formulation comprising: a first
oligosiloxane-modified nano-phase segregating liquid crystalline
material and at least one additional material selected from a
second oligosiloxane-modified nano-phase segregating liquid
crystalline material, non-liquid crystalline oligosiloxane-modified
materials, organic liquid crystalline materials, or organic
non-liquid crystalline materials, wherein the liquid crystal
formulation is nano-phase segregated in the SmC* phase, has an
I.fwdarw.SmC* phase transition, with SmC* temperature range from
about 15.degree. C. to about 35.degree. C., has a tilt angle of
about 22.5.degree. or about 45.degree..+-.6.degree., has a
spontaneous polarization of less than about 50 nC/cm.sup.2, and has
a rotational viscosity of less than about 600 cP; wherein one or
more of the first or second oligosiloxane-modified nano-phase
segregating liquid crystalline materials is a phenyl benzoate, a
biphenyl, a terphenyl, or a phenyl pyrimidine, such that the phenyl
pyrimidine has a formula ##STR00046## where a=0 or 1; c=0 or 1; f=0
or 1; h=0 or 1; i=0-4; k=0, 1, or 2; p=0, 1, or 2; with the proviso
that if f=0, c=0; with the proviso that if a=0, h=0; Y is
independently selected from halogen, NO.sub.2, CN, CH.sub.3,
CF.sub.3, or OCF.sub.3; X=an alkyl or a substituted alkyl with at
least one chiral centre, where individual chiral groups can be
racemic or non-racemic, provided that the individual chiral groups
are selected so that the liquid crystal formulation is non-racemic;
and V is ##STR00047## with n=3-15; d=1-5; and R' and R'' are
independently selected from C.sub.rH.sub.(2r+1) and r=1-4, or a
phenyl group; R is an alkyl group having from 1 to 10 carbon atoms
or one of W, W', or W''; wherein W is ##STR00048## with n'=3-15;
a'=0 or 1; m=1 or 2; s=1 or 2; q=0 or 1; b=0 or 1; i'=0-4; T=O,
COO, OCO, CH.dbd.N, N.dbd.CH, CF.sub.2O, OCF.sub.2, NHCO, CONH,
CH.sub.2, CH.sub.2CH.sub.2, C.ident.C, --CH.dbd.CH--, or
CF.sub.2CF.sub.2; Y' is independently selected from halogen,
NO.sub.2, CN, CH.sub.3, CF.sub.3, OCF.sub.3; Q=O, COO, or OCO; and
X'=an alkyl; or substituted alkyl with at least one chiral centre,
where individual chiral groups can be racemic or non-racemic,
provided that the individual chiral groups are selected so that the
liquid crystal formulation is non-racemic; W' is ##STR00049## with
n''=3-15; a''=0 or 1; b'=0 or 1; L is independently selected from
H, halogen, NO.sub.2, CN, CH.sub.3, CF.sub.3, OCF.sub.3; Q'.dbd.O,
COO or OCO; and X''=an alkyl; or substituted alkyl with at least
one chiral centre, where individual chiral groups can be racemic or
non-racemic, provided that the individual chiral groups are
selected so that the liquid crystal formulation is non-racemic; and
W'' is one of ##STR00050## with n'''=3-15; q=0 or 1; p'=0, 1, or 2;
k' is 0,1, or 2; i''=0-4; t is 0 or 1; u=0 or 1; with the proviso
that when t=0, u=0, Y'' is independently selected from halogen,
NO.sub.2, CN, CH.sub.3, CF.sub.3, or OCF.sub.3; E is an alkyl; or
substituted alkyl with at least one chiral centre, where individual
chiral groups can be racemic or non-racemic, provided that the
individual chiral groups are selected so that the liquid crystal
formulation is non-racemic; the terphenyl has a formula
##STR00051## where a=0 or 1; b=0 or 1; L is independently selected
from halogen, NO.sub.2, CN, CH.sub.3, CF.sub.3 or OCF.sub.3; Q=O,
COO, or OCO; X=an alkyl, or substituted alkyl with at least one
chiral centre, where individual chiral groups can be racemic or
non-racemic, provided that the individual chiral groups are
selected so that the liquid crystal formulation is non-racemic; and
A is ##STR00052## with n=3-15; d=1-5; and R' and R'' are
independently selected from C.sub.rH.sub.(2r+1) and r=1-4, or a
phenyl group; R is an alkyl group having from 1 to 10 carbon atoms
or one of W, W', or W''; wherein W is ##STR00053## with n'=3-15;
a'=0 or 1; m=1 or 2; s=1 or 2; q=0 or 1; b'=0 or 1; i=1-4; T=O,
COO, OCO, CH.dbd.N, N.dbd.CH, CF.sub.2O, OCF.sub.2, NHCO, CONH,
CH.sub.2, CH.sub.2CH.sub.2, C.ident.C, --CH.dbd.CH--, or
CF.sub.2CF.sub.2; Y is independently selected from halogen,
NO.sub.2, CN, CH.sub.3, CF.sub.3, OCF.sub.3; Q'.dbd.O, COO, or OCO;
and X'=an alkyl; or substituted alkyl with at least one chiral
centre, where individual chiral groups can be racemic or
non-racemic, provided that the individual chiral groups are
selected so that the liquid crystal formulation is non-racemic; W'
is ##STR00054## with n''=3-15; a''=0 or 1; b''=0 or 1; L' is
independently selected from H, halogen, NO.sub.2, CN, CH.sub.3,
CF.sub.3, OCF.sub.3; Q''.dbd.O, COO, or OCO; and X''=an alkyl; or
substituted alkyl with at least one chiral centre, where individual
chiral groups can be racemic or non-racemic, provided that the
individual chiral groups are selected so that the liquid crystal
formulation is non-racemic; and W'' is one of ##STR00055## with
n'''=3-15; q=0 or 1; p=0, 1, or 2; k is 0, 1, or 2; l'=0-4; t is 0
or 1; u=0 or 1; with the proviso that when t=0, u=0, Y' is
independently selected from halogen, NO.sub.2, CN, CH.sub.3,
CF.sub.3, or OCF.sub.3; E is an alkyl; or substituted alkyl with at
least one chiral centre, where individual chiral groups can be
racemic or non-racemic, provided that the individual chiral groups
are selected so that the liquid crystal formulation is non-racemic;
and the phenyl benzoate or biphenyl has a formula ##STR00056##
where a=0 or 1; m=1 or 2; s=1 or 2; q=0 or 1; b=0 or 1; l=0-4; T=O,
COO, OCO, CH.dbd.N, N.dbd.CH, CF.sub.2O, OCF.sub.2, NHCO, CONH,
CH.sub.2, CH.sub.2CH.sub.2, C.ident.C, --CH.dbd.CH--, or
CF.sub.2CF.sub.2; Y is independently selected from halogen,
NO.sub.2, CN, CH.sub.3, CF.sub.3, or OCF.sub.3; Q=O, COO, or OCO;
X=an alkyl; or substituted alkyl with at least one chiral centre,
where individual chiral groups can be racemic or non-racemic,
provided that the individual chiral groups are selected so that the
liquid crystal formulation is non-racemic; and A is ##STR00057##
with n=3-15; d=1-5; and R' and R'' are independently selected from
C.sub.rH.sub.(2r+1) and r=1-4, or a phenyl group; R is an alkyl
group having from 1 to 10 carbon atoms or the group W; wherein W is
##STR00058## with n'=3-15; a'=0 or 1; m'=1 or 2; s'=1 or 2; q'=0 or
1; b'=0 or 1; l'=0-4; T'=O, COO, OCO, CH.dbd.N, N.dbd.CH,
CF.sub.2O, OCF.sub.2, NHCO, CONH, CH.sub.2, CH.sub.2CH.sub.2,
C.ident.C, --CH.dbd.CH--, or CF.sub.2CF.sub.2; Y' is independently
selected from halogen, NO.sub.2, CN, CH.sub.3, CF.sub.3, OCF.sub.3;
Q'=O, COO, or OCO; and X'=an alkyl; or substituted alkyl with at
least one chiral centre, where individual chiral groups can be
racemic or non-racemic, provided that the individual chiral groups
are selected so that the liquid crystal formulation is
non-racemic.
2. A liquid crystal formulation comprising: a first
oligosiloxane-modified nano-phase segregating liquid crystalline
material and at least one additional material selected from a
second oligosiloxane-modified nano-phase segregating liquid
crystalline material, non-liquid crystalline oligosiloxane-modified
materials, organic liquid crystalline materials, or organic
non-liquid crystalline materials, wherein the liquid crystal
formulation is nano-phase segregated in the SmC* phase, has an
I.fwdarw.SmC* phase transition, with SmC* temperature range from
about 15.degree. C. to about 35.degree. C., has a tilt angle of
about 22.5.degree. or about 45.degree..+-.6.degree., has a
spontaneous polarization of less than about 50 nC/cm.sup.2, and has
a rotational viscosity of less than about 600 cP; wherein the
additional material has a formula: ##STR00059## where e=0 or 1; G
is H, halogen, an epoxide, NO.sub.2, CN, CH.sub.3, CF.sub.3, or
OCF.sub.3; M is an alkyl; substituted alkyl with at least one
chiral centre, where individual chiral groups can be racemic or
non-racemic, provided that the individual chiral groups are
selected so that the liquid crystal formulation is non-racemic; or
the group ##STR00060## where n=3-15; d=1-5; and R' and R'' are
independently selected from C.sub.rH.sub.(2r+1) and r=1-4, or a
phenyl group; R is an alkyl group having from to 10 carbon atoms,
or Z, where Z is ##STR00061## where n'=3-15; e'=0 or 1; G' is H,
halogen, an epoxide, NO.sub.2, CN, CH.sub.3, CF.sub.3, or
OCF.sub.3.
3. A liquid crystal formulation comprising: a first
oligosiloxane-modified nano-phase segregating liquid crystalline
material and at least one additional material selected from a
second oligosiloxane-modified nano-phase segregating liquid
crystalline material, non-liquid crystalline oligosiloxane-modified
materials, organic liquid crystalline materials, or organic
non-liquid crystalline materials, wherein the liquid crystal
formulation is nano-phase segregated in the SmC* phase, has an
I.fwdarw.SmC* phase transition, with SmC* temperature range from
about 15.degree. C. to about 35.degree. C., has a tilt angle of
about 22.5.degree. or about 45.degree..+-.6.degree., has a
spontaneous polarization of less than about 50 nC/cm.sup.2, and has
a rotational viscosity of less than about 600 cP; wherein the
additional material has a formula of one of ##STR00062## where r=0
or 1; p=0, 1, or 2; v=0, 1, or 2; x=0 or 1; q=0 or 1; i=0-4; with
the when r=0, x=0; Y is independently selected from halogen,
NO.sub.2, CN, CH.sub.3, CF.sub.3, or OCF.sub.3; J and J' are
independently selected from an alkyl; or substituted alkyl with at
least one chiral centre, where individual chiral groups can be
racemic or non-racemic, provided that the individual chiral groups
are selected to ensure that the liquid crystal formulation is
non-racemic.
4. The liquid crystal formulation of claim 1 wherein the
spontaneous polarization is less than about 40 nC/cm.sup.2.
5. The liquid crystal formulation of claim 1 wherein the liquid
crystal formulation has an I.fwdarw.SmC*.fwdarw.SmX phase
transition.
6. The liquid crystal formulation of claim 1 wherein the first or
second oligosiloxane-modified nano-phase segregating liquid
crystalline material has an ABA structure.
7. The liquid crystal formulation of claim 1 wherein the liquid
crystal formulation has a birefringence of more than about
0.05.
8. A device containing a liquid crystal formulation of claim 1, the
device having a stable bookshelf geometry, bistable switching, and
isothermal electric field alignment in the SmC* phase, the device
having a response time of less than 500 .mu.s when switched between
two stable states, and an electric drive field of less than about
30 V/.mu.m.
9. The device of claim 8 comprising: at least one liquid crystal
cell comprising: a pair of substrates having a gap therebetween; a
pair of electrodes, the pair of electrodes positioned on one of the
substrates or one electrode positioned on each substrate; and the
liquid crystal formulation of claim 1 disposed in the gap between
the pair of substrates.
10. The device of any of claim 9 further comprising at least one
polarizer.
11. The device of claim 9 wherein the device further comprises a
rubbed alignment layer.
12. The device of claim 11 wherein the alignment layer is a
polyimide based material.
13. The device of claim 10 wherein the alignment layer has a
thickness less than 200 nm.
14. A device of claim 10 having response time of less than about
100 .mu.s when switched between two stable states.
15. The device of claim 10 wherein the electric drive field is less
than about 10 V/.mu.m.
16. The device of claim 10 wherein the device has a contrast ratio
of at least 10:1 in the case where the tilt angle is 22.5
degrees.+-.6 degrees.
17. The device of claim 10 wherein the device has a relaxation of
less than about 10% in transmission intensity after 20 ms after
removal of the electric drive field.
Description
This application relates to the use of oligosiloxane modified
liquid crystals and their use in electro-optic devices. The
invention specifically relates to the formulation of such liquid
crystals to enable their use in bistable, ferroelectric displays
which can be isothermally electric field aligned, and which also
have very low Spontaneous polarizations (Ps) which are required for
practical devices utilizing active matrix backplane
technologies.
Thermotropic liquid crystals are materials which are capable of
exhibiting liquid crystal, or mesogenic phases, where the phase can
change as a function of temperature. The liquid crystalline phases,
such as nematic, or smectic, tend to exist between the isotropic
and crystalline phases and exhibit physical properties which are
not observed for isotropic (liquid) or crystalline phases. For
example, a liquid crystal phase can exhibit both birefringent and
fluid behaviors at the same temperature. Such properties have been
exploited in electro-optic devices such as transmissive and
reflective displays, where the birefringence can be effectively
tuned by the application of electric fields in a device structure
where the orientation of the liquid crystal molecules has been
controlled. Nematic liquid crystals have been widely exploited in
liquid crystal displays (LCD's), for example in displays for laptop
computers, cell phones, PDAs, computer monitors, and TVs. While
electro-optic devices based upon nematic liquid crystals have been
widely utilized, the fastest response time of such devices is
restricted to on the order of a millisecond, because the devices
rely on a surface alignment controlled relaxation process for part
of the switching cycle. Ferroelectric liquid crystals have the
potential to switch between optical states much more rapidly.
However, although both digital and analogue mode devices have been
developed, such devices have proven to be difficult to deploy and
therefore have only been commercialized in specialized, micro
display applications such as camera viewfinders.
Clark and Lagerwall (U.S. Pat. No. 4,367,924, and Applied Physics
Letters, 36, 899-901, (1980), both of which are incorporated herein
by reference) have described devices which utilize organic
ferroelectric liquid crystals which exhibit sub-microsecond
electro-optic switching speeds. The Clark and Lagerwall devices are
so-called Surface Stabilized Ferroelectric Liquid Crystal Devices
(SSFLCDs). Such devices utilize organic ferroelectric liquid
crystals, or their formulations, which exhibit the chiral smectic C
(SmC*) phase that is required for the digital switching SSFLCD
mode. The materials typically exhibit the following phase sequence
upon cooling in order to facilitate the manufacture of SSFLCDs:
Isotropic.fwdarw.Nematic.fwdarw.SmA*.fwdarw.SmC*, where SmA* is the
chiral smectic A phase. This phase sequence permits the formation
of surface stabilized aligned phases due to the surface
registration of the liquid crystalline molecules in the low
viscosity nematic, higher temperature phase. The aligned liquid
crystal device is then carefully cooled through the SmA* phase and
into the SmC* phase to create the SSFLCD. If the SmC* phase can be
robustly aligned into the so-called `bookshelf` geometry, then the
devices exhibit bistable ferroelectric switching.
However, this has proved to be difficult in practice. SSFLCDs are
susceptible to several problems which have resulted in only limited
commercialization of the technology. A key limitation results from
the phase sequence employed because conventional, organic FLCs
undergo significant layer shrinkage during the transition when
cooled from the higher temperature SmA* into the lower temperature
SmC* phase. The shrinkage of the layered structures results in the
formation of defects (zig-zag defects, due to the formation of
buckled layers, or chevrons) which significantly reduce the
contrast ratios observed for SSFLCDs. The formation of chevron
structures and the control of these structures enable the
fabrication of either C1 or C2 type devices, as is well known to
those skilled in the art, for example, see Optical Applications of
Liquid Crystals, Ed. L Vicari, Chapter 1, ISBN 0750308575. In some
cases, the ideal so-called "bookshelf geometry," where the layers
of the SmC* phase are arranged perpendicular to the device
substrates and alignment layers, can be induced in such materials
by the application of an electric field. However, devices with
induced, or pseudo, bookshelf structures are not practical for
commercial display devices due to manufacturing requirements and
the potential for the devices to revert to chevron alignment once
deployed. Thus, while many SSFLCD patents claim that bookshelf
structures are present, it is important to understand whether such
structures are true bookshelf structures or pseudo bookshelf
structures, and whether chevron structures are present when
utilized for devices. These limitations of conventional SSFLCDs are
also discussed by Crossland et al. in Ferroelectrics, 312, 3-23
(2004).
This inherent problem for FLC materials with the
Isotropic.fwdarw.Nematic SmA*.fwdarw.SmC* phase sequence has led to
the investigation of new materials which are not prone to the layer
shrinkage phenomenon. One approach to eliminate this is to use so
called `de Vries` materials which exhibit an
Isotropic.fwdarw.SmA*.fwdarw.SmC* phase sequence and where there is
practically no layer shrinkage at the SmA*.fwdarw.SmC* phase
transition. The absence of a very low viscosity nematic phase
requires alternative alignment schemes to allow the random domains
and natural helielectric state of the SmC* phase to be converted
into a phase structure approaching a mono-domain, which is
orientated with respect to the electrodes and substrates to yield a
practical electro-optic device.
Coles (U.S. Pat. No. 5,498,368 and Proceedings of SPIE, Vol. 2408,
22-29 (1995), both of which are incorporated herein by reference)
highlighted the unexpected properties of oligosiloxane-modified
ferroelectric liquid crystals based upon phenylbenzoate aromatic
cores. True bistability, i.e., the retention of the
electrically-selected orientation of the LC mono-domain after the
removal of an applied electric field, and the greatly reduced
sensitivity of the FLC tilt angle over temperature ranges as wide
as 50.degree. C., were demonstrated in this patent. In this case, a
mono-domain was created by slowly cooling the device (e.g.,
1.degree. C./min) from the isotropic phase and then through the
SmC* in the presence of an applied electric field. Crossland et al.
(WO 2005019380A1, incorporated herein by reference) later
demonstrated devices using simple, single component oligosiloxane
FLCs based upon phenyl benzoate aromatic cores which utilized only
electric fields for mono-domain alignment, and which were described
as being bistable, based upon the definition included in the patent
application.
Goodby et al. (U.S. Publication 2005/0001200A1, incorporated herein
by reference) described a composition of matter for a class of
oligosiloxane liquid crystal containing a biphenyl core. Goodby
noted that such materials can be used alone or in an admixture with
other liquid crystals, although he did not discuss the design of
such mixtures beyond the use of claimed materials which each has a
SmA phase to stabilize the SmA phase of the resulting liquid
crystal mixture. Based on this and the comparative compound
examples within the patent it is apparent that the intent is to
design conventional SSFLC mixtures with the
Isotropic.fwdarw.Nematic.fwdarw.SmA*.fwdarw.SmC* phase sequence.
The patent discussed only the phase sequences of the materials
claimed, with no mention of other critical physical properties
which are needed to construct a practical FLCD.
Li et al. (J. Mater. Chem., 17, 2313-2318, (2007), incorporated
herein by reference) prepared some achiral siloxane terminated
phenylpyrimidines. Some of these materials had an
Isotropic.fwdarw.SmC.fwdarw.Crystal phase sequence (mesogens 1a,
1b, 1c, 1d, 1e, 2e, 5, 6, 7, 8 in the table below), while others
had an Isotropic.fwdarw.SmA.fwdarw.SmC.fwdarw.Crystal phase
sequence (mesogens 2a, 2b, 2c, 2d, 3, 4 in the table below). He
used 1 mole % of a chiral oligosiloxane ("Br11-Si.sub.3") as an
additive to mesogens 1b, 2b, 3, 4, 5, 6, 7, and 8 in an attempt to
measure the optical tilt angle by POM (Polarized Optical
Microscopy). He noted that others had observed discrepancies
between the X-ray data and POM observations for siloxane-terminated
liquid crystals and investigated the relationship between the
smectic layer spacing defined by X-ray and the optical tilt angle
of selected mesogens. The phase sequences of the binary mixtures
formed are not reported. He reported that five mixtures (based upon
1b, 5, 6, 7, and 8, all of which have an Isotropic.fwdarw.SmC phase
sequence) were prepared but could not be aligned into a
mono-domain, and that he could not measure a tilt angle. He noted
the alignment materials and the cell gap used, but did not discuss
the process used to attempt to create alignment within the test
cell. He noted that he was able to align one sample, based on
mesogen 2b, and a tilt angle of 36 degrees was measured. This tilt
angle is not useful for a practical FLCD, where tilt angles close
to 22.5 degrees or 45 degrees are a prerequisite depending on the
operational mode of the FLC device. He noted that samples must be
aligned to measure the tilt angle and reported tilt angles for two
further mixtures based upon mesogens 3 and 4 (24 and 26 degrees
respectively). Thus, he only reported that he could align mixtures
where the chiral additive was added to a mesogen with an
Isotropic.fwdarw.SmA.fwdarw.SmC.fwdarw.Crystal phase sequence. The
abstract and summary highlight the bone fide de Vries behavior of
mesogen 3, which has a terminal chlorine atom and an
Isotropic.fwdarw.SmA*.fwdarw.SmC* phase sequence. The structures
are shown below.
##STR00001##
Walba et al. (U.S. Pat. No. 6,870,163, incorporated herein by
reference) noted that it is well known to those skilled in the art
of FLC materials and devices that a typical FLC device does not
exhibit true optical bistability due to chevron defect formation.
Crossland et al., in Ferroelectrics, 312, 3-23 (2004) (incorporated
herein by reference), discuss the impact of this limitation on
device operation, for example, the need for DC balancing and
inverse framing, leading to `dead periods` during imaging. U.S.
Pat. No. 6,507,330 (Handschy et al.) also discussed the need for DC
balancing.
In WO2005/019380A1 (incorporated herein by reference), Crossland et
al. noted the unique properties of oligosiloxane FLCs and devices,
including electric field alignment, insensitivity of the tilt angle
to temperature within the SmC* phase, and true bistability,
combined with the ability to rotate the aligned smectic mono-domain
with respect to rubbing direction of the alignment layers within
the device. However, such features were only demonstrated for
single component, phenylbenzoate based oligosiloxane mesogens. It
was noted that the tilt angle can only be tuned by changing the
molecular structure of the component, i.e., all the required
properties must be designed into a single molecular structure.
Those skilled in the LC art know that molecules are usually
formulated to provide mixtures with broad operating ranges and to
tune the many physical properties which must be optimized to meet
the requirements of a practical FLC device. The vast majority of
this formulation knowledge has been developed using organic FLCs
which have been developed for use in the conventional mode, chevron
devices which also utilize materials with the
Isotropic.fwdarw.Nematic.fwdarw.SmA*.fwdarw.SmC* phase
sequence.
Oligosiloxane modified liquid crystals are differentiated from
conventional liquid crystals due to their propensity to form
nano-phase segregated layered structures, as described by Coles et
al. (Liquid Crystals, 23(2), 235-239, (1997); J. Phys II France, 6,
271-279, (1996)) and Li et al. (J. Mater. Chem., 17, 2313-2318,
(2007) and references cited therein, all of which are incorporated
herein by reference). Such systems have been described as "virtual
polymers" because their structures and properties combine some of
the features of Side Chain Liquid Crystal Polymers (SCLCP) and some
of the properties of conventional organic liquid crystals. The
structure and properties of oligosiloxane modified liquid crystals
differ so significantly from organic liquid crystals that they have
been classified as a type of amphiphilic, or nano-phase segregated,
liquid crystal in a recent scientific review article. (see C.
Tschierske, `Non-conventional liquid crystals--the importance of
micro-segregation for self-organization`, J. Mater. Chem., 1998,
8(7), 1485-1508). The structures of such systems are still an area
of active scientific debate, see Li et al. (J. Mater. Chem., 17,
2313-2318, (2007), all of which are incorporated herein by
reference).
The formulation of oligosiloxane-modified, nano-segregated
ferroelectric liquid crystals for use in practical devices, for
example, including but not restricted to, active matrix
Ferroelectric LCDs (FLCDs), has not been studied in detail. The
formulation of organic liquid crystals has been extensively
studied, and many predictive rules have been developed to aid the
design of the liquid crystal phase behavior of such formulations
(Demus et al., Mol. Cryst. Liq. Cryst., 25, 215-232, (1974); Hsu et
al., Mol. Cryst. Liq. Cryst., 27, 95-104, (1974); Rabinovich et
al., Ferroelectrics, 121, 335-342, (1991)). However, in our
experience, such formulation design approaches are not suitable for
oligosiloxane FLCs because even standard "rules of thumb" that the
phase of an unknown liquid crystal can be identified if it is
miscible with a liquid crystal with a known phase (Goodby &
Gray, in Physical Properties of Liquid Crystals, ISBN
3-527-29747-2, page 17), i.e., "like liquid crystals" are miscible
with "like liquid crystals," break down. Such basic rules do not
apply to oligosiloxane modified ferroelectric liquid crystals where
the nano-phase segregated smectic layering dominates and other
classes of liquid crystal, or even non-liquid crystal molecules,
are readily admixed without the loss of the smectic phase
structure. For example, Coles and Li have independently
demonstrated unexpected examples of miscibility in such systems,
highlighting the difference of oligosiloxane systems from organic
LC systems (see Coles et al., Ferroelectrics, 243, 75-85, (2000)
and Li et al., Advanced Materials 17(5), 567-571, (2005), both of
which are incorporated herein by reference). Prior to the present
invention, well-defined predictive rules for the formulation of
compositions containing high levels of oligosiloxane liquid
crystals have not been identified, nor has the ability to tune
physical property sets to meet practical device materials,
alignment and robustness requirements been demonstrated. For
example, the attempt of Li et al. (J. Mater. Chem., 17, 2313-2318,
(2007)) to study the tilt angle of a simple series of materials was
frustrated because only three of the eight mixtures prepared could
even be aligned to allow the tilt angle to be determined.
Canon (U.S. Pat. No. 5,720,898, incorporated herein by reference),
describes a class of device containing a main chain type liquid
crystal containing a siloxane linking group, and a liquid
crystalline monomer. In U.S. Pat. No. 5,720,898, the smallest main
chain polymer can be an ABA species, where A=a mesogenic group and
B=a disiloxane linkage. This patent teaches that the smectic ABA
material is added as a minor component to a monomeric, organic
mesogen and there is no suggestion that the liquid crystal phase is
nano-phase segregated. In fact, the siloxane additive does not
perturb the conventional smectic phase structure. The inventors
noted that the phase can be stabilized provided the covalently
bonded ABA oligomer is able to span adjacent layers of the smectic
phase. The liquid crystal system is macroscopically aligned by
stretching or shearing of the LC medium within the device. In this
example, the layer structure is not nano-phase segregated because
it is based on monomeric, organic mesogens, and the ABA
oligosiloxane is added at low concentration to span the existing
layers, thus pinning them together and stabilizing the phase. The
patent teaches that if the siloxane linking segment is too large,
the molecule may fold into a hairpin and no longer span the
adjacent layers, and thus the pinning mechanism is lost.
Therefore, there is a need for formulations of oligosiloxane liquid
crystal materials which can be used in bistable, ferroelectric
displays.
The present invention meets that need by providing a nano-phase
segregated oligosiloxane modified liquid crystal formulation with a
balanced property set for application in practical devices. The
liquid crystal formulation comprises a first oligosiloxane-modified
nano-phase segregating liquid crystalline material; and at least
one additional material selected from a second
oligosiloxane-modified nano-phase segregating liquid crystalline
material, non-liquid crystalline oligosiloxane-modified materials,
organic liquid crystalline materials, or organic non-liquid
crystalline materials, wherein the liquid crystal formulation is
nano-phase segregated in the SmC* phase, has an I.fwdarw.SmC* phase
transition, with a SmC* temperature range from about 15.degree. C.
to about 35.degree. C., has a tilt angle of about
22.5.degree..+-.6.degree. or about 45.degree..+-.6.degree., and has
a spontaneous polarization of less than about 50 nC/cm.sup.2, and a
rotational viscosity of less than about 600 cP.
Another aspect of the invention is a device containing a liquid
crystal formulation. The device has a stable bookshelf geometry,
bistable switching, and isothermal electric field alignment, a
response time of less than 500 .mu.s when switched between two
stable states, and an electric drive field of less than about 30
V/.mu.m.
FIG. 1 shows a cross-section of a typical bistable liquid crystal
cell.
FIG. 2 is a graph showing the tilt angle of a formulation as a
function of the amount of a compound having an
I.fwdarw.SmA.fwdarw.Cr phase sequence.
FIG. 3 is a graph showing the temperature dependence of tilt angle
for formulations having different ratios of materials having
I.fwdarw.SmC* and I.fwdarw.N.fwdarw.SmA*.fwdarw.SmC* phase
sequences.
FIGS. 4a and 4b are graphs showing drive voltage and optical
transmission as a function of time.
FIG. 5a is a graph showing the temperature dependence of tilt angle
and FIG. 5b is a graph showing drive voltage and optical
transmission as a function of time.
FIG. 6 is a graph showing drive voltage and optical transmission as
a function of time.
FIG. 7 is a graph showing the temperature dependence of tilt
angle.
FIG. 8 is a graph showing drive voltage and optical transmission as
a function of time.
FIG. 9 is a graph showing the temperature dependence of tilt
angle.
We have determined that the behaviors of formulated
oligosiloxane-modified liquid crystals are fundamentally different
from the majority of conventional liquid crystals due to the
nano-segregation. Furthermore, we have shown distinct features
stemming from the presence of the resultant siloxane rich region in
the layers. For example, the oligosiloxane modification has been
found to promote the formation of the smectic phase due to
nano-segregation. In addition, because of the impact of
nano-segregated smectic layering, other classes of liquid crystals
and non-liquid crystal molecules are readily admixed without the
loss of the smectic phase structure. These are important features
because of the challenge in achieving necessary property sets in a
single molecule. Therefore, property optimization by mixing of
various components is an important approach in realizing practical
liquid crystal materials. The stabilized smectic phase found in a
distinct class of liquid crystals, represented herein by the
nano-segregating oligosiloxane-modified liquid crystals, is an
important feature in the present invention where applications
focused formulation is employed to realize a practical composition
with a well balanced property set, while retaining the chiral
smectic phase structure necessary for ferroelectric liquid crystal
properties. This approach helps to achieve practical FLC devices.
Prior to the present invention, well-defined predictive rules for
the formulation of compositions containing high levels of
oligosiloxane liquid crystals, demonstrating the ability to tune
physical property sets to meet practical device materials, have not
been demonstrated. The present invention shows the benefit of the
use of oligosiloxane-modified liquid crystal as a base liquid
crystal composition to formulate a stable ferroelectric liquid
crystal composition with a balanced property set that can be
utilized to realize practical devices based on Si-TFT technology.
Furthermore tailored device structures and practical alignment
schemes have been developed for these oligosiloxane-modified liquid
crystal formulations, which eliminates significant fabrication and
alignment stability issues for conventional
Isotropic.fwdarw.Nematic.fwdarw.SmA*.fwdarw.SmC* organic
ferroelectric liquid crystals, which are known to those skilled in
the art.
The present invention will demonstrate how to develop the basic
materials and device properties required for practical devices
within nano-phase segregated, oligosiloxane FLC systems
successfully. Formulations having an Isotropic.fwdarw.SmC* phase
sequence and the novel ferroelectric devices that they enable are
the subject of the present patent application. Although wholly
organic mesogens may be formulated with this phase sequence, the
present application relates to oligosiloxane FLCs. These low
molecular mass liquid crystals are hybrid siloxane-organic
moieties, where a discreet siloxane segment is grafted onto an
organic moiety, or moieties, in an AB or ABA fashion, where
B=oligosiloxane and A=organic. The siloxane is oligomeric and is
thus differentiated from Side-Chain Liquid Crystal Polysiloxanes
(SCLCP), Main-Chain Liquid Crystal Polysiloxanes (MCLCP), or Liquid
Crystal polysiloxane Elastomers (LCE) in both structure and
physical properties. Oligosiloxane LCs are of interest because they
combine stable smectic phases with the high degree of mobility
required for the operation of practical LCDs.
The present invention relates to the design of optimized
ferroelectric liquid crystal formulations which contain at least
one oligosiloxane-modified liquid crystalline material. The
oligosiloxane-modified liquid crystalline material may be blended
with other oligosiloxane-modified liquid crystals, organic liquid
crystals, non-liquid crystalline hybrid oligosiloxane organic
materials, or non-liquid crystalline organic materials to create
formulations with optimized liquid crystalline properties. The
formulations may be used to prepare FLC devices which are electric
field aligned and exhibit true bistability. These features enable
digital addressing schemes without the need to use inverse frames
for the purposes of DC-balancing, coupled with the ability to
align, or re-align, the device isothermally, at any time, using
electric fields. The latter property overcomes the short-comings of
Isotropic.fwdarw.Nematic.fwdarw.SmA*.fwdarw.SmC* phase sequence
materials, where the requirement for slow cooling makes it
difficult to re-align a device that has damaged alignment caused by
mechanical shock or temperature excursions once it has been
deployed. Optionally, the formulations which are the subject of
this application may exhibit phases below the SmC* phase (i.e., at
lower temperature) where the electric field aligned texture is
retained and truly bistable switching is observed upon heating back
into the SmC* without any significant impact on the operation of
the device, for example, a reduction of the contrast ratio of the
device. The properties of devices fabricated using the claimed
formulations and device fabrication methods utilized result from
the unique nano-phase segregated structures of the
oligosiloxane-modified liquid crystals which form the base of the
formulations. The oligosiloxane-modified liquid crystalline
component(s) should always be present in sufficient concentration
to induce a nano-phase segregated SmC* phase, for example, as
detected by X-Ray Diffraction studies.
The formulation includes at least two components. There can one or
more oligosiloxane-modified liquid crystalline components in the
formulation. In addition, there can be one or more non-liquid
crystalline oligosiloxane-modified materials, organic liquid
crystalline materials, or organic non-liquid crystalline materials
in the formulation. The components which are not
oligosiloxane-modified liquid crystalline components (if any) are
generally present in an amount of less than about 50 mol %, or less
than about 45 mol %, or less than about 40 mol %, or less than
about 35 mol %, or less than about 30 mol %.
These formulations are designed for use in a range of devices which
utilize amplitude or phase modulation of light including, but not
limited to, transmissive displays, spatial light modulators, and
reflective mode microdisplays. Such devices may utilize passive
matrix style addressing or active pixel addressing with thin film
transistors (TFT) backplanes, for example, devices such as Passive
Matrix Liquid Crystal Devices (PMLCD), or Active Matrix Liquid
Crystal Devices (AMLCD). In this application, we will focus upon
the case of AMCLD devices, which can operate in transmissive or
reflective modes. However, the formulations are not intended to be
limited to use with such a device; they could be used with other
types of devices, which are well known to those of skill in the
art. The use of TFTs to control liquid crystal orientation, whether
based upon amorphous silicon (a-Si), Low Temperature
Polycrystalline Silicon (LTPS), or crystalline Silicon, imposes
constraints on the magnitude of the spontaneous polarization (Ps)
of the liquid crystal formulation which can be tolerated due to
charge transport limitations of the TFT. A low Ps value
considerably simplifies the design of the TFT-based Active Matrix.
Those skilled in the art will be aware that a high Ps results in
reduced degrees of freedom within display design, for example,
lower resolution, smaller display size and potentially reduced
aperture sizes, and ultimately preclude the use of Si-TFT.
Simplified backplane circuitry enables larger aperture ratios
(i.e., brighter displays) and lower cost.
The formulations of the present invention are specifically designed
to have low spontaneous polarization (Ps values) to enable them to
be used in active matrix backplane electro-optic devices. If the Ps
value is too high, then the current flow produced during the
electric field induced re-orientation of the mesogens from one
optical state to the other exceeds the plausible design space for
the pixel circuitry's current driving capacity. As is well known to
those skilled in the art, the Ps can be either positive or
negative. When values are given in this application, the number is
intended to mean both the positive and the negative value. For
example, a Ps of 10 nC/cm.sup.2 means either +10 nC/cm.sup.2 or -10
nC/cm.sup.2.
The electro-optic response time of a ferroelectric liquid crystal
may be determined by the following equation:
.tau..varies..eta./PsE
where
.tau.=the time required for the optical response to change from 10%
to 90%.
E=the applied electric field which drives the change in the optical
states
Ps=the spontaneous polarization
.eta.=the rotational viscosity.
In practice, the response time should be as fast as possible, and
preferably < about 500 microseconds, or < about 250
microseconds, or < about 100 microseconds, or < about 75
microseconds or < about 50 microseconds. The magnitude of the Ps
of the formulation is limited by the backplane (for example, <
about 50 nC/cm.sup.2, or < about 40 nC/cm.sup.2, or < about
30 nC/cm.sup.2, or < about 20 nC/cm.sup.2), and the electric
field necessary for switching should be as low as possible (for
example, < about 30 V/.mu.n, or < about 20 V/.mu.m, or <
about 15 V/.mu.m, or < about 10 V/.mu.m, or < about 5
V/.mu.m). In addition to developing FLC formulations with
Isotropic.fwdarw.SmC* phase sequences on cooling, there is a need
to minimize the rotational viscosities to optimize the
electro-optic response times for the low Ps systems (for example,
< about 600 cP, or < about 400 cP, or < about 300 cP, or
< about 200 cP, or < about 100 cP, or < about 50 cP).
Previous applications (for example, the Crossland (WO 2005/019380)
and Dow Corning (US2007/009035) applications) highlighted single
component ferroelectric liquid crystals. However, the single
component materials were not optimized for AMLCD. In practice, it
is very difficult to design a single molecule which exhibits all
the attributes required for use in AMLCD. The present invention
provides methods to optimize these attributes via a formulation
approach, which are more suited for use in AMLCD.
For example, in the case of a practical transmissive AMLCD, the
careful design of formulations based upon oligosiloxane-modified
liquid crystalline material(s) and the custom design of a suitable
design primitive enable the formulations to demonstrate a number of
desirable features. By "design primitive" we mean the integration
of a liquid crystal formulation with suitable substrates, alignment
layer technology, electrode structures, and polarizer technologies
that are required to fabricate a basic FLC electro-optic device.
Such devices are differentiated from existing ferroelectric liquid
crystals devices by a combination of the composition of the
formulation, the liquid crystal phase sequences, and the alignment
properties. Favorable features for both AMCLD and PMLCD include: 1)
A wide SmC* phase and, therefore, wide FLC operating temperature
range, spanning ambient temperature. By wide we mean at least
spanning about 15.degree. C. to about 35.degree. C. and preferably
about 10.degree. C. to about 40.degree. C., or about 0.degree. C.
to about 50.degree. C., or about -20.degree. C. to about 60.degree.
C., or about -30.degree. C. to about 80.degree. C. 2) An alignment
process which allows the formation of a liquid crystalline
mono-domain, or near mono-domain, with a bookshelf geometry within
the design primitive. The alignment process can be undertaken
within the SmC* phase of formulated, nano-phase segregated,
Isotropic.fwdarw.SmC* systems, isothermally using suitable electric
fields. This differs from the FLCD prior art, where specific
overlying liquid crystal phases (specifically SmA* and Nematic) and
a carefully controlled cooling profile through the
Isotropic.fwdarw.Nematic.fwdarw.Smectic A* and eventually into the
SmC* phase is essential. The ability to align the SmC* phase
isothermally is advantageous, simplifying device fabrication and
allowing alignment to be achieved without the need to design
complex phase sequences in the formulation. The ability to use
isothermal, electric field alignment in the SmC* phase enables the
device to be re-aligned at will during deployment, which is of
great significance, as those skilled in the art will know that
current ferroelectric liquid crystal devices may irreversibly lose
alignment due to mechanical shock or temperature excursions where
the liquid crystal becomes crystalline or isotropic. 3) The
resulting bookshelf structure should be stable during the operation
and storage of the device. In cases where some degradation is
observed, then the isothermal, electric alignment scheme employed
for oligosiloxane ferroelectric liquid crystal formulations can be
used to repair the alignment. Many conventional, all organic FLCs
have claimed bookshelf, or pseudo bookshelf geometries, but these
structures are not stable enough for deployment in devices. The
bookshelf structures claimed here have enhanced integral stability
within the design primitive. We have discovered that the enabling
effect of the nano-phase segregated oligosiloxane-modified liquid
crystalline molecules, as described for single component systems by
Coles, Crossland, and Dow Corning, can be retained in suitably
formulated systems. The nano-phase segregated bookshelf structure
of a dual segment host stabilizes the structure. The pinning
mechanism described by Canon is not required in nano-phase
segregated oligosiloxane liquid crystal systems, and we have
demonstrated the ability to achieve true bistability in systems
which do not contain ABA (i.e., bi-mesogenic) species. Thus, the
tri-segment (ABA) molecules used by Canon are not required for the
stabilization of the formulations described here. However,
tri-segment molecules may be used in the broadening of the SmC*
temperature range in the present invention, if desired.
Formulations are also designed to eliminate the formation of
chevron defects by eliminating an overlying Smectic A phase,
resulting in formulations with a direct I.fwdarw.SmC* phase
transition. A potential failure mode of conventional organic FLCDs
is the loss of alignment if the FLC material is allowed to
crystallize at low temperature, for example during storage or
shipping. We have demonstrated that formulations can be developed
which do not crystallize. These formulations have a wide SmX phase
below the SmC* phase. The SmX phase is defined as a non-crystalline
phase in which electro-optic switching ceases under the conditions
defined herein, but in which the macroscopic molecular alignment of
the bookshelf structure is retained at low temperature. Although
the device is not operational in this phase, it becomes operational
again when allowed to return to the operational temperature range.
4) The alignment quality and uniformity should be sufficient to
enable the realization of high contrast ratios and bistability over
the entire active area of a device. By high contrast, we mean
equivalent or superior to commercial organic
Isotropic.fwdarw.Nematic.fwdarw.Smectic A*.fwdarw.SmC* phase
sequence formulations tested under equivalent conditions. 5) The
tilt angle should be tuned to a specific value for the efficient
operation of polarizer based electro-optic devices. For example, in
the case of transmissive devices the optimum tilt angle is 22.5
degrees, .+-.6 degrees, or 22.5 degrees, .+-.4 degrees, or 22.5
degrees, .+-.2 degrees. Furthermore, the tilt angle should not
change too dramatically within the operational temperature range of
the device. The ability to design formulations with a range of tilt
angles is also advantageous; for example, formulations with a tilt
angle of 45 degrees, .+-.6 degrees, or 45 degrees, .+-.4 degrees,
or 45 degrees, .+-.2 degrees, can also be used for phase modulating
devices. 6) The need for a low Ps has been noted above. Although a
low Ps is a requirement of the TFT-based Active Matrix backplane
technologies as currently exploited in commercial LCDs, this
imposes a significant challenge for devices whose alignment is
undertaken in a viscous smectic phase at, or near, ambient
temperature using electric field alignment protocols. In addition
to the alignment process, lower Ps can negatively impact response
time of the liquid crystal device at fixed temperature and driving
field. 7) For digital mode devices, true bistability is a
requirement. By "true bistability", we mean the retention of the
optical signal, within a specific tolerance, for some time after
the removal of the switching field. An example of tolerance is that
the optical signal should not degrade by more than about 20%, or by
more than 10%, or by more than 5%. A short term relaxation to a
plateau value may be acceptable, but a continuous decline in
optical transmission is not acceptable. The acceptable time is
dictated by the application and by the drive architecture, and can
range from minutes to milliseconds. 8) The birefringence of the
formulation should be optimized based upon the design primitive,
i.e., the AMLCD design. The birefringence is typically greater than
about 0.05, or greater than about 0.1. The birefringence should not
vary widely over the operational temperature range, for example the
variation in birefringence of < about 100 ppm/.degree. C., or
< about 50 ppm/.degree. C. between the lower end of the
operational temperature range and about 5.degree. C. below the
SmC*.fwdarw.Isotropic phase transition.
Practical FLC devices can be developed if formulations are designed
which operate within the constraints defined above. As noted
previously, while a considerable body of formulation experience
exists for organic FLC systems based upon organic materials, such
information cannot be directly transferred to the present
oligosiloxane-based FLC formulations because of the combined impact
of the following: i) the increased structural complexity of the
nano-phase segregated structure exhibited by the oligosiloxane
based systems covered herein; ii) the utilization of a specific
phase sequence for the vast majority of organic FLCs, i.e.,
Isotropic.fwdarw.Nematic.fwdarw.SmA*.fwdarw.SmC* for organic
systems; iii) the ability to observe reduced temperature dependence
of Ps and tilt angle in oligosiloxane-based formulations; iv) the
electric field alignment and layer rotation features of
oligosiloxane-based formulations; v) the true bistability of
oligosiloxane-based formulations; vi) the ability to tune tilt
angle in nano-phase segregated systems; vii) the ability to design
sub-SmC* phase properties which can avoid the disruption of the
preferred molecular alignment at low temperatures; and viii) the
ability to suppress nematic phase formation in
oligosiloxane-modified ferroelectric liquid crystal formulations,
for example, when 4-n-pentyl-4'-cyanobiphenyl (compound 9) or Felix
15/000 (`compound` 15) are added to smectic oligosiloxane
systems.
One approach is to design formulations with an
Isotropic.fwdarw.SmC*.fwdarw.Crystal or preferably an
Isotropic.fwdarw.SmC*.fwdarw.SmX phase sequence. We have discovered
that materials with a wide range of phase behaviors can be used to
develop formulations with the above phase sequences. Materials with
phase sequences selected from, but not limited to, the following
types can be used in formulation: i) Isotropic.fwdarw.SmC*; ii)
Isotropic.fwdarw.SmA; iii) Isotropic.fwdarw.SmA.fwdarw.SmC; iv)
Isotropic.fwdarw.SmA*.fwdarw.SmC*; v) Isotropic.fwdarw.Nematic; vi)
monotropic liquid crystalline phases; vii) non liquid crystalline
materials; etc. Not all of the materials used for formulation need
to be oligosiloxane functionalized, provided there is sufficient
oligosiloxane modified material present to preserve the nano-phase
segregated structure in the formulation.
In one embodiment of the invention, the properties of an
I.fwdarw.SmC* phase sequence oligosiloxane liquid crystal are tuned
in the following manner. 1) The aromatic core is selected to reduce
inter-molecular interactions, thus lowering the rotational
viscosity of the final formulation. 2) The hydrocarbon chain
separating the aromatic core from the siloxane is selected to
provide optimum decoupling from the oligosiloxane, while providing
a low regime (about 22.5 degrees) or high regime (about 45 degree)
tilt angle. 3) The oligosiloxane is selected to be as short as
possible to obtain the maximum possible birefringence, while
maintaining the required phase properties. 4) A smectic A material
can be added to reduce the effective tilt angle of the formulation,
without inducing a SmA phase in the formulation. 5) Several
approaches can be taken to achieve a low overall Ps value. For
examples, a mesogenic species of intrinsically low Ps can be made,
achiral and chiral species can be formulated to set a Ps, or
materials with opposing optical activity can be formulated to tune
Ps.
Our investigations have shown that the selection and optimization
of such formulations involves balancing the effects of different
components. For example, an additive which is effective at reducing
the tilt angle may not be as effective in reducing the rotational
viscosity, or it may hinder the alignment of the sample.
Oligosiloxane-modified nano-phase segregating liquid crystalline
materials used in the preparation of suitable formulations include,
but are not limited to, the structures given below. Note that the
oligosiloxane-modified nano-phase segregating liquid crystalline
materials can be defined as AB (two segment adduct) or ABA (three
segment adduct, also known as an LC dimer), where B=the siloxane
segment and A=the aromatic liquid crystal core. ABA' structures are
also given, where A and A' are non equivalent groups, leading to
asymmetric structures.
I) Components which can be Use to Create the Nano-Phase Segregated
Smectic Phase (Generic Structures)
Among the oligosiloxane-modified liquid crystalline materials which
can be used to create the nano-phase segregated smectic phase in
the formulation are phenylbenzoates and biphenyls, terphenyls, and
phenylpyrimidines. Examples of suitable materials are shown
below.
1) Phenylbenzoates and Biphenyls
One class of compounds has the formula:
##STR00002## where a=0 or 1; m=1 or 2; s=1 or 2; q=0 or 1; b=0 or
1; i=0-4; T=O, COO, OCO, CH.dbd.N, N.dbd.CH, CF.sub.2O, OCF.sub.2,
NHCO, CONH, CH.sub.2, CH.sub.2CH.sub.2, C.ident.C., --CH.dbd.CH--
or CF.sub.2CF.sub.2; Y is independently selected from halogen,
NO.sub.2, CN, CH.sub.3, CF.sub.3, OCF.sub.3; Q=O, COO, or OCO; and
X=an alkyl; or a substituted alkyl with at least one chiral centre,
where individual chiral groups can be racemic or non-racemic,
provided that the individual chiral groups are selected so that the
liquid crystal formulation is non-racemic; where, A is
##STR00003## where n=3-15; d=1-5; R', and R'' are independently
selected from C.sub.rH.sub.(2r+1), and r=1 to 4, or a phenyl group;
R is an alkyl group having from 1 to 10 carbon atoms or the group
W, where W is
##STR00004## where n=3-15; a=0 or 1; m=1 or 2; s=1 or 2; q=0 or 1;
b=0 or 1; i=0-4; T=O, COO, OCO, CH.dbd.N, N.dbd.CH, CF.sub.2O,
OCF.sub.2, NHCO, CONH, CH.sub.2, CH.sub.2CH.sub.2, C.ident.C,
--CH.dbd.CH--, or CF.sub.2CF.sub.2; Y is independently selected
from halogen, NO.sub.2, CN, CH.sub.3, CF.sub.3, OCF.sub.3; Q=O,
COO, or OCO; and X=an alkyl; or a substituted alkyl with at least
one chiral centre, where individual chiral groups can be racemic or
non-racemic, provided that the individual chiral groups are
selected so that the liquid crystal formulation is non-racemic.
The alkyl and substituted alkyl groups represented by X typically
have from 2 to 20 carbon atoms. The substituted alkyls can be
substituted with one or more of the following groups: further alkyl
groups, halogens, epoxides, NO.sub.2, CN, CF.sub.3, or
OCF.sub.3.
2) Terphenyls
Another class of suitable compounds is terphenyls having the
formula:
##STR00005## where a=0 or 1; b=0 or 1; L is independently selected
from H, halogen, NO.sub.2, CN, CH.sub.3, CF.sub.3, OCF.sub.3; Q=O,
COO, or OCO; and X=an alkyl; or a substituted alkyl with at least
one chiral centre, where individual chiral groups can be racemic or
non-racemic, provided that the individual chiral groups are
selected so that the liquid crystal formulation is non-racemic;
where A is
##STR00006## where n=3-15; d=1 to 5; R' and R'' are independently
selected from C.sub.rH.sub.(2r+1) and r=1 to 4, or a phenyl group;
where R is an alkyl group having from 1 to 10 carbon atoms, or one
of W'' or W, as defined elsewhere, or W', where W' is
##STR00007## where n=3-15; a=0 or 1; b=0 or 1; L=is independently
selected from H, halogen, NO.sub.2, CN, CH.sub.3, CF.sub.3,
OCF.sub.3; Q=O, COO, or OCO; and X=an alkyl; or a substituted alkyl
with at least one chiral centre, where individual chiral groups can
be racemic or non-racemic, provided that the individual chiral
groups are selected so that the liquid crystal formulation is
non-racemic.
The alkyl and substituted alkyl groups represented by X typically
have from 2 to 20 carbon atoms. The substituted alkyls can be
substituted with one or more of the following groups: further alkyl
groups, halogens, epoxides, NO.sub.2, CN, CF.sub.3, or
OCF.sub.3.
3) Phenyl Pyrimidines
Other classes of suitable compounds are phenyl (or
biphenyl)pyrimidines having the formulas:
##STR00008## where a=0 or 1, p=0, 1 or 2, k=0, 1 or 2, f=0 or 1;
h=0 or 1; c=0 or 1; i=0-4; with the proviso that if f=0, c=0; with
the proviso that if a=0, h=0; Y is a halogen, NO.sub.2, CN,
CH.sub.3, CF.sub.3, or OCF.sub.3; where X=an alkyl; or a
substituted alkyl with at least one chiral centre, where individual
chiral groups can be racemic or non-racemic, provided that the
individual chiral groups are selected so that the liquid crystal
formulation is non-racemic; where V is
##STR00009## where n=3-15; d=1-5; and R' and R'' are independently
selected from C.sub.rH.sub.(2r+1) and r=1-4, or a phenyl group;
where R is an alkyl group having from 1 to 10 carbon atoms, or W,
or W', as defined elsewhere, or W'', where W'' is selected from one
of the following groups to create a symmetrical or asymmetrical
dimeric additive:
##STR00010## where n=3-15; g is 0 or 1; p is 0, 1 or 2; k is 0, 1
or 2; i=0-4; t is 0 or 1; u=0 or 1; with the proviso that when t=0,
u=0; Y is independently selected from halogen, NO.sub.2, CN,
CH.sub.3, CF.sub.3, or OCF.sub.3; E is an alkyl; or a substituted
alkyl with at least one chiral centre, where individual chiral
groups can be racemic or non-racemic, provided that the individual
chiral groups are selected so that the liquid crystal formulation
is non-racemic.
The alkyl and substituted alkyl groups represented by X and E
typically have from 2 to 20 carbon atoms. The substituted alkyls
can be substituted with one or more of the following groups:
further alkyl groups, halogens, epoxides, NO.sub.2, CN, CF.sub.3,
or OCF.sub.3.
II) Components which can be Use to Tune the Properties of the
Nano-Phase Segregated Smectic Phase (Generic Structures)
The following classes of materials are useful as additives to
formulations containing the oligosiloxane-modified nano-phase
segregating liquid crystalline materials given above.
##STR00011## where e=0 or 1; G is H, a halogen, an epoxide,
NO.sub.2, CN, CH.sub.3, CF.sub.3, or OCF.sub.3; M is an alkyl; or a
substituted alkyl with at least one chiral centre, where individual
chiral groups can be racemic or non-racemic, provided that the
individual chiral groups are selected so that the liquid crystal
formulation is non-racemic; or the group
##STR00012## where n=3-15; d=1-5; and R' and R'' are independently
selected from C.sub.rH.sub.(2r+1) and r=1-4, or a phenyl group; R
is an alkyl group having from 1 to 10 carbon atoms, or Z, where Z
is
##STR00013## where n=3-15; e=0 or 1; G is H, a halogen, an epoxide,
NO.sub.2, CN, CH.sub.3, CF.sub.3, or OCF.sub.3.
The alkyl and substituted alkyl groups represented by M typically
have from 2 to 20 carbon atoms. The substituted alkyls can be
substituted with one or more of the following groups: further alkyl
groups, halogens, epoxides, NO.sub.2, CN, CF.sub.3, or
OCF.sub.3.
The following classes of materials may also be used as
additives.
##STR00014## where r=0 or 1; p=0, 1 or 2; v=0, 1, or 2; x can be 0
or 1, q=0 or 1; i=0-4; with the proviso that when r=0, x=0; Y is
independently selected from halogen, NO.sub.2, CN, CH.sub.3,
CF.sub.3, or OCF.sub.3; J and J' are independently selected from an
alkyl; or a substituted alkyl with at least one chiral centre,
where individual chiral groups can be racemic or non-racemic,
provided that the individual chiral groups are selected to ensure
that the liquid crystal formulation is non-racemic.
The alkyl and substituted alkyl groups represented by J and J'
typically have from 2 to 20 carbon atoms. The substituted alkyls
can be substituted with one or more of the following groups:
further alkyl groups, halogens, epoxides, NO.sub.2, CN, CF.sub.3,
or OCF.sub.3.
If the oligosiloxane-modified nano-phase segregating liquid
crystalline components are achiral, then organic chiral molecules
can also be used to induce chirality in the liquid crystal
formulation.
Examples of Formulations
Liquid crystals molecules (mesogens) are routinely formulated into
complex mixtures. Such formulations enable property sets to be
realized which would be difficult, or even impossible, to realize
from a single molecule. The Crossland (WO 2005/019380) and Dow
Corning patent applications (US 2007/009035) identified single
component systems which exhibited electric field alignment and
bistable switching; however, such molecules require formulation if
they are to be used in wide temperature and active matrix backplane
devices. The development of formulated systems based upon
oligosiloxane-modified liquid crystals is complicated by the
unusual micro-phase segregated nature of such materials. The
examples given below illustrate how the phase sequence, temperature
range of the SmC* phase, spontaneous polarization (Ps), and tilt
angle may be controlled in such systems. The formulation of such
materials can not be extrapolated from examples of organic FLCs,
because the nano-phase segregated oligosiloxane region, which is
absent in organic FLC systems, plays an important role in
controlling the properties of the bulk formulation, and the
electro-optic properties of devices fabricated from them.
The chemical structures of the components used in the different
formulations are shown in Table 1. The formulations and their
properties are shown in Tables 2-8. Table 2 shows the phase
behavior of cyanobiphenyl based materials used for tilt angle
tuning. Table 3 shows data for examples of binary formulations
based upon an oligosiloxane-modified terphenyl mesogen and organic
cyanobiphenyl mesogens. Table 4 shows examples of binary, ternary
and quaternary formulations based upon an oligosiloxane-modified
terphenyl mesogen and an oligosiloxane-modified cyanobiphenyl
mesogen. Table 5 shows examples of formulations containing multiple
oligosiloxane-modified terphenyl mesogens. Table 6 shows examples
of formulations containing an oligosiloxane-modified
phenylpyrimidine and a chiral oligosiloxane phenylpyrimidine
dopant. Table 7 shows examples of formulations containing an
oligosiloxane-modified phenylpyrimidines and various chiral
oligosiloxane modified dopants. Table 7 shows examples of ternary
formulations containing an oligosiloxane-modified phenylpyrimidines
and a chiral oligosiloxane phenylpyrimidine dopant. Table 8 shows
examples of miscellaneous formulations.
Formulations were prepared by weighing components into a vessel and
then heating the vessel to a temperature about 10.degree. C. above
the clearing temperature (liquid crystal to isotropic transition),
or melting point in the case of a non liquid crystalline component,
of the component with the highest transition temperature for the
formation of an isotropic phase. Samples were held and mixed at
this temperature for about 5-10 minutes, and were then allowed to
cool down to ambient temperature. All compositions are listed as
the mole percentage of each component unless otherwise stated.
Formulations were initially characterized using a Differential
Scanning Calorimeter (DSC). The temperature range of the DSC
experiment was typically -40.degree. C. to 120.degree. C., unless
the clearing phase transition temperature of the formulation was
>100.degree. C., in which case the upper temperature was
increased. Fresh samples were heated into the isotropic phase
(Heating run #1), then cooled to -40.degree. C. (Cooling run #1),
then heated back into the isotropic phase (Heating run #2) then
cooled back to -40.degree. C. (Cooling run #2), then heated back
into the isotropic phase (Heating run #3), then cooled back to room
temperature (Cooling run #3). Heating runs #2 and #3 were used to
define the phase transition temperatures, by selecting the peak
temperature for each transition. Thermo-optic analysis using a
polarizing optical microscope and a programmable hot stage system
was undertaken in order to classify the type of liquid crystal
phase present. The current reversal method as described by Miyasato
et al., Japan Journal Applied Physics, 22, L661, (1983) for
determining Ps was used to confirm the presence of an SmC* phase,
and to identify the transition temperature boundaries of the SmC*
phase. The thermo-optic and electro-optic measurements were
undertaken in single pixel devices which were constructed using ITO
glass substrates, separated with spacer beads and edge sealed with
adhesive. Rubbed polyimide alignment layers were used in the
devices. See FIG. 1.
FIG. 1 shows the structure of a typical bistable liquid crystal
cell used to test the formulations. The liquid crystalline
formulation 17 is placed between two substrates 10, 11. The
substrates can be made of any suitable material, such as glass,
silicon, organic polymers, or inorganic polymers, for example. One
or both of the substrates can be transparent, depending on the
class of device.
The inner surfaces of the substrates 10, 11 have electrodes 12, 13,
e.g., aluminum or indium tin oxide (ITO), which can be applied in
selected regions. One electrode can be on each substrate, or both
electrodes can be on one of the substrates (but only one pair of
electrodes is required). One or both of the electrodes can be
transparent, depending on the device. Alternatively, there can be
electrodes providing fringing fields, enabling the electro-optic
effects to be controlled. The inner surface of the electrode may be
coated with a passivation layer, if desired.
The inner surface of the electrode (adjacent to the liquid crystal
material), or the substrate in the case of the fringing field
device, is coated with alignment layers 14, 15 in order to
facilitate the electric field alignment, the layer orientation and
the switching of the SmC* phase. The alignment layer can be an
organic coating, or an inorganic coating. Suitable alignment layers
include, but are not limited to, polyamide, polyimide, polyester,
polytetrafluoroethylene, silicon oxides, silanes, and polysilanes.
However, the exact choice of alignment layer material and its
preparation conditions are important to realize good alignment and
bistability, although the exact selections are dependent on the
composition of the formulations. Preferred materials include
polyimides with pre-tilt angles of < about 3 degrees; however
other materials may also be used. Examples of materials which can
be used include polyimides sold under the designations SE130,
SE1410, SE8292, and RN1199, available from Nissan Chemical
Industries. The alignment layer can be formed by any method known
in the art, including, but not limited to, rubbing, stretching,
deposition, and embossing. The alignment layer helps the monodomain
to form (i.e., "the bookshelf"), and bistable switching to be
observed. In order to achieve uniform alignment and bistability,
the thickness of alignment layer should be < about 200 nm, or
< about 100 nm, or < about 50 nm, or <25 nm.
Spacers 16 separate the substrates 10, 11, and define the cell
thickness. A sealing layer 18 is used to retain the liquid crystal
material in the cell. The liquid crystal electro-optic devices of
the present invention typically have a cell gap designed to be in
the range of 0.5 microns to 10 microns.
The laminated device can be placed between polarizers 19, 20
oriented at 90 degrees to each other (optic axis) to generate
bright or dark states when the liquid crystal is switched between
two states. The device described in FIG. 1 is a transmission mode
device. Alternative polarizer configurations, known to those
skilled in the art, may be used for transmission and reflective
mode devices.
TABLE-US-00001 TABLE 1 Chemical structures of components used in
formulations. Compound Number Structure C1 ##STR00015## C2
##STR00016## C3 ##STR00017## C4 ##STR00018## C5 ##STR00019## C6
##STR00020## C7 ##STR00021## C8 ##STR00022## C9 ##STR00023## C10
##STR00024## C11 ##STR00025## C12 ##STR00026## C13 ##STR00027## C14
##STR00028## C15 Commercial organic FLC formulation purchased from
AZ Electronics (Felix015/000). C16 ##STR00029## C17 ##STR00030##
C18 ##STR00031## C19 ##STR00032## C20 ##STR00033## C21 ##STR00034##
C22 ##STR00035## C23 ##STR00036## C24 ##STR00037## C25 ##STR00038##
C26 ##STR00039## C27 ##STR00040## C28 ##STR00041##
TABLE-US-00002 TABLE 2 Phase behavior of Cyanobiphenyl based
materials used for tilt angle tuning. Compound Phase Behavior
##STR00042## Crystal .fwdarw. 48.degree. C. .fwdarw. SmA .fwdarw.
58.5.degree. C. .fwdarw. Isotropic ##STR00043## Crystal .fwdarw.
42.degree. C. .fwdarw. SmA .fwdarw. 48.degree. C. Nematic .fwdarw.
49.5.degree. C. .fwdarw. Isotropic ##STR00044## Crystal .fwdarw.
24.degree. C. .fwdarw. Nematic .fwdarw. 35.3.degree. C. .fwdarw.
Isotropic ##STR00045## Crystal .fwdarw. 37.0.degree. C. .fwdarw.
SmA .fwdarw. 59.0.degree. C. .fwdarw. I BDH Data Sheet
851/PP/2.0/0686 M. Ibn-Elhaj et al. J. Phys. II France, 1807-1817
(1993).
TABLE-US-00003 TABLE 3 Data for Binary formulations based upon an
oligosiloxane-modified terphenyl mesogen and organic cyanobiphenyl
mesogens. Formula- Composition tion (by mole Tilt Angle Rotational
Number percentage) Phase Sequence (degrees) Ps (nC/cm.sup.2)
Viscosity/cP 1 C1:100 SmX .fwdarw. 37.6 .fwdarw. SmC* .fwdarw. 85.5
.fwdarw. I 39 (@40.degree. C.) 60 (@40.degree. C.) 950 (@40.degree.
C.) 2 C1:90 SmX .fwdarw. 32.7 .fwdarw. SmC* .fwdarw. 92.4 .fwdarw.
I 31 (@40.degree. C.) 51 (@40.degree. C.) 400 (@40.degree. C.)
C7:10 3 C1:83 SmX .fwdarw. 28.9 .fwdarw. SmC* .fwdarw. 74.8
.fwdarw. SmA .fwdarw. 95.5 .fwdarw. I 23 (@40.degree. C.) 35
(@40.degree. C.) 120 (@40.degree. C.) C7:17 4 C1:75 SmX .fwdarw.
24.2 .fwdarw. SmA .fwdarw. 97.7 .fwdarw. I NA NA NA C7:25 5 C1:83
SmX .fwdarw. 27.0 .fwdarw. SmC* .fwdarw. 74.2 .fwdarw. SmA .fwdarw.
96.2 .fwdarw. I 22 (@40.degree. C.) 31 (@40.degree. C.) 127
(@40.degree. C.) C8:17 6 C1:90 SmX .fwdarw. 32.0 .fwdarw. SmC*
.fwdarw. 93.4 .fwdarw. I 31 (@40.degree. C.) 46 (@40.degree. C.)
555 (@40.degree. C.) C9:10 7 C1:90 SmX .fwdarw. 33.5 .fwdarw. SmC*
.fwdarw. 90.3 .fwdarw. I 34 (@40.degree. C.) 48 (@40.degree. C.) --
C10:10 8 C1:83 SmX .fwdarw. 28.5 .fwdarw. SmC* .fwdarw. 85.2
.fwdarw. SmA .fwdarw. 93.4 .fwdarw. I 31 (@40.degree. C.) 45
(@40.degree. C.) 900 (@40.degree. C.) C10:17 9 C1:75 SmX .fwdarw.
31.0 .fwdarw. SmC* .fwdarw. 74.7 .fwdarw. SmA .fwdarw. 94.3
.fwdarw. I 25 (@40.degree. C.) 32 (@40.degree. C.) 250 (@40.degree.
C.) C10:25 10 C1:87 SmX .fwdarw. 31.3 .fwdarw. SmC* .fwdarw. 83.5
.fwdarw. I 36 (@40.degree. C.) 50 (@40.degree. C.) 465 (@40.degree.
C.) C15:13.dagger. .dagger.N.B. Weight % used for this blend,
because C15 is a pre-formulated liquid crystal additive. [See Table
1 for chemical structures of individual components].
TABLE-US-00004 TABLE 4 Data for binary, ternary and quaternary
formulations based upon an oligosiloxane- modified terphenyl
mesogen and an oligosiloxane-modified cyanobiphenyl mesogen.
Formula- Composition tion (by mole Tilt Angle Rotational Number
percentage) Phase Sequence (degrees) Ps (nC/cm.sup.2) Viscosity/cP
11 C1:90 SmX .fwdarw. 33.5 .fwdarw. SmC* .fwdarw. 90.3 .fwdarw. I
34 (@40.degree. C.) 48 (@40.degree. C.) -- C10:10 12 C1:48.8 SmX
.fwdarw. 16.4 .fwdarw. SmC* .fwdarw. 66.9 .fwdarw. I 34
(@40.degree. C.) 17.7 (@40.degree. C.) -- C10:16.2 C22:35 13 C1:49
SmX .fwdarw. 16.5 .fwdarw. SmC* .fwdarw. 75.0 .fwdarw. I 28
(@40.degree. C.) 22 (@40.degree. C.) 395 (@40.degree. C.) C10:16
28.8 (@25.degree. C.) 19 (@25.degree. C.) 795 (@25.degree. C.)
C22:17 C16:18
TABLE-US-00005 TABLE 5 Data for formulations containing multiple
oligosiloxane-modified terphenyl mesogens Formula- Composition tion
(by mole Tilt Angle Rotational Number percentage) Phase Sequence
(degrees) Ps (nC/cm.sup.2) Viscosity/ cP 14 C1:50 SmX .fwdarw. 33.3
.fwdarw. SmC* .fwdarw. 82.5 .fwdarw. I 40 (@40.degree. C.) 53
(@40.degree. C.) 580 (@40.degree. C.) C4:50 15 C1:33 SmX .fwdarw.
14.5 .fwdarw. SmC* .fwdarw. 84.3 .fwdarw. I 40.5 (@25.degree. C.)
95 (@25.degree. C.) 1880 (@25.degree. C.) C2:33 C3:33 16 C1:25 SmX
.fwdarw. 11.8 .fwdarw. SmC* .fwdarw. 61.4 .fwdarw. I -- 18
(@25.degree. C.) -- C4:25 20 (@40.degree. C.) C22:25 C16:25
TABLE-US-00006 TABLE 6 Data for formulations containing an
oligosiloxane-modified phenylpyrimidine and a chiral oligosiloxane
phenylpyrimidine dopant. Formula- Composition tion (by mole Tilt
Angle Rotational Number percentage) Phase Sequence (degrees) Ps
(nC/cm.sup.2) Viscosity/cP 17 C17:95 SmX .fwdarw. 22.0 .fwdarw.
SmC* .fwdarw. 52.5 .fwdarw. I 23 (@25.degree. C.) 3 (@25.degree.
C.) 118 (@25.degree. C.) C23:5 18 C17:90 SmX .fwdarw. -29.7
.fwdarw. SmC* .fwdarw. 51.7 .fwdarw. I 26 (@25.degree. C.) 10
(@25.degree. C.) 147 (@25.degree. C.) C23:10 19 C17:85 SmX .fwdarw.
-29.2 .fwdarw. SmC* .fwdarw. 50.5 .fwdarw. I 27 (@25.degree. C.) 16
(@25.degree. C.) 175 (@25.degree. C.) C23:15 24 (@40.degree. C.) 11
(@40.degree. C.) 56 (@40.degree. C.) 20 C17:75 SmX .fwdarw. -26.8
.fwdarw. SmC* .fwdarw. 48.7 .fwdarw. I 30.5 (@25.degree. C.) 30
(@25.degree. C.) 135 (@25.degree. C.) C23:25 21 C17:50 SmX .fwdarw.
18.8 .fwdarw. SmC* .fwdarw. 85.6 .fwdarw. I 36 (@25.degree. C.) 82
(@25.degree. C.) 230 (@25.degree. C.) C23:50 22 C18:90 SmX .fwdarw.
24.7 .fwdarw. SmC* .fwdarw. 58.7 .fwdarw. I 27.5 (@40.degree. C.) 8
(@40.degree. C.) 118 (@40.degree. C.) C23:10 23 C19:85 SmX .fwdarw.
39.0 .fwdarw. SmC* .fwdarw. 57.8 .fwdarw. I 26 (@40.degree. C.) 17
(@40.degree. C.) 100 (@40.degree. C.) C23:15 24 C20:85 Cr .fwdarw.
41.0 .fwdarw. SmC* .fwdarw. 56.7 .fwdarw. I -- 18 (@25.degree. C.)
SC -- C23:15 25 C21:85 Cr .fwdarw. 41.5 .fwdarw. SmC* .fwdarw. 60.7
.fwdarw. I 30 (@40.degree. C.) SC 17 (@40.degree. C.) SC 166
(@40.degree. C.) C23:15 26 C17:83.3 SmX .fwdarw. -30 .fwdarw. SmC*
.fwdarw. 51.5 .fwdarw. I 27.5 (@25.degree. C.) 19 (@25.degree. C.)
195 (@25.degree. C.) C5:1.7 C23:15 27 C17:76.5 SmX .fwdarw. 5
.fwdarw. SmC* .fwdarw. 51.2 .fwdarw. I 29 (@25.degree. C.) 14
(@25.degree. C.) 278 (@25.degree. C.) C1:8.5 C23:15 28 C17:76.5 SmX
.fwdarw. 19.0 .fwdarw. SmC* .fwdarw. 57.5 .fwdarw. I 23
(@25.degree. C.) 9 (@25.degree. C.) 295 (@25.degree. C.) C24:8.5
C23:15 29 C17:76.5 SmX .fwdarw. -33.5 .fwdarw. SmC* .fwdarw. 45.1
.fwdarw. I 29 (@25.degree. C.) 15 (@25.degree. C.) 181 (@25.degree.
C.) C25:8.5 C23:15 NB SC = supercooled sample.
TABLE-US-00007 TABLE 7 Data for formulations containing
oligosiloxane-modified phenylpyrimidines and various chiral
oligosiloxane modified dopants Formula- Composition tion (by mole
Tilt Angle Rotational Number percentage) Phase Sequence (degrees)
Ps (nC/cm.sup.2) Viscosity/cP 30 C17:50 SmX .fwdarw. 15.2 .fwdarw.
SmC* .fwdarw. 60.6 .fwdarw. I 35 (@40.degree. C.) 23 (@40.degree.
C.) 278 (@40.degree. C.) C1:50 31 C22:50 SmX .fwdarw. 12.8 .fwdarw.
SmC* .fwdarw. 59.7 .fwdarw. I 44.5 (@40.degree. C.) 40 (@40.degree.
C.) 686 (@40.degree. C.) C11:50 32 C17:80 SmX .fwdarw. 4.0 .fwdarw.
SmC* .fwdarw. 51.8 .fwdarw. I 35 (@25.degree. C.) 12 (@25.degree.
C.) -- C12:20
TABLE-US-00008 TABLE 8 Data for miscellaneous formulations Formula-
Composition tion (by mole Tilt Angle Rotational Number percentage)
Phase Sequence (degrees) Ps (nC/cm.sup.2) Viscosity/cP 33 C17:54.8
SmX -35.2 SmC* 55.0 I 39 (@25.degree. C.) 43 (@25.degree. C.) 399
(@25.degree. C.) C28:20.2 C23:25 34 C1:23 SmX 17.5 SmC* 61.2 I 41
(@25.degree. C.) 13 (@25.degree. C.) 650 (@25.degree. C.) C13:77 35
C1:50 SmX 17.2 SmC* 87.4 I 40 (@40.degree. C.) 19 (@40.degree. C.)
320 (@40.degree. C.) C13:25 C6:25 36 C14:67.5 SmX -13.3 SmC* 51.4 I
27 (@25.degree. C.) 6 (@40.degree. C.) 42 (@40.degree. C.)
C15:22.5.dagger. C1:10 37 C26:66 Cr 15.3 SmC* 35.9 I 20
(@30.degree. C.) 29 (@30.degree. C.) 319 (@30.degree. C.) C27:33
.dagger.N.B. Weight % used for this blend, because C15 is a
pre-formulated liquid crystal additive.
EXAMPLE 1
An oligosiloxane liquid crystal C17 was formulated with a
non-liquid crystalline oligosiloxane C23. C17 exhibits
I.fwdarw.SmA.fwdarw.SmC.fwdarw.Cr phase behavior while C23 is a
non-liquid crystalline compound. The binary formulations were found
to exhibit I.fwdarw.SmC*.fwdarw.SmX phase behavior, illustrating
the unexpected ability to obtain the desired I.fwdarw.SmC* phase
behavior from components with different phase behaviors.
TABLE-US-00009 Composition (by Tilt Angle mole percentage) Phase
Sequence (degrees) Ps (nC/cm.sup.2) C17:100 Cr .fwdarw. 16.9
.fwdarw. SmC .fwdarw. 45.6 .fwdarw. SmA .fwdarw. 54.3 .fwdarw. I NA
NA C17:95 SmX .fwdarw. 22.0 .fwdarw. SmC* .fwdarw. 52.5 .fwdarw. I
23 (@25.degree. C.) 3 (@25.degree. C.) C23:5 C17:90 SmX .fwdarw.
-29.7 .fwdarw. SmC* .fwdarw. 51.7 .fwdarw. I 26 (@25.degree. C.) 10
(@25.degree. C.) C23:10 C17:85 SmX .fwdarw. -29.2 .fwdarw. SmC*
.fwdarw. 50.5 .fwdarw. I 27 (@25.degree. C.) 16 (@25.degree. C.)
C23:15 C17:75 SmX .fwdarw. -26.8 .fwdarw. SmC* .fwdarw. 48.7
.fwdarw. I 30.5 (@25.degree. C.) 30 (@25.degree. C.) C23:25 C17:50
SmX .fwdarw. 18.8 .fwdarw. SmC* .fwdarw. 85.6 .fwdarw. I 36
(@25.degree. C.) 82 (@25.degree. C.) C23:50 C23:100 Cr .fwdarw.
50.3 .fwdarw. I NA NA
EXAMPLE 2
C1 with I.fwdarw.SmC* phase sequence was mixed at various ratios
with C10 which has I.fwdarw.SmA.fwdarw.K phase sequence. Two
formulations with different amounts of C10 were prepared. Although
C10 only exhibits a SmA phase, all formulations exhibited SmC*
phase.
TABLE-US-00010 Response Time C1:C10 (mole ratio) Tilt Angle
(.degree.) (.mu.s) 83:17 30.5 200 75:25 25 50
The electro-optical properties of these formulations were measured
in a 13 mm.times.16 mm liquid crystal cells depicted in FIG. 1. The
liquid crystal test cells were prepared in the following manner: an
ITO coating was photo-patterned with 5 mm.times.5 mm active area
with a contact pad for each. ITO coated glass had a SiO.sub.2
coating between glass substrate and the ITO coating, and the sheet
resistance of ITO was the 100 ohm/square. A designated alignment
agent was spin coated to a thickness of about 25 nm, cured, and
then rubbed to form the alignment layer. Spacers of the desired
size were blended with UV curable sealant at about 2% (by weight)
loading, and this was applied at two edges of a cell on one of the
substrates, on top of the alignment layer. It was laminated with
another substrate without sealant application with the alignment
layers facing inside and with an anti-parallel rubbing orientation.
The two substrates were assembled in staggered fashion with 13
mm.times.13 mm substrates overlap and 5 mm.times.5 mm counter
facing electrodes and with two opposing 3 mm ledges with contact
pads for connection to electrical source. The assembly was pressed
using vacuum press and irradiated with a UV light source to cure
the sealant.
A transmissive liquid crystal device was prepared by filling a cell
prepared using nylon as the alignment layer and 3 .mu.m spacers
with aforementioned formulations. The ports were then sealed with
UV curable sealant and wires were attached by soldering to contact
pads for the opposing ITO electrodes to apply an electric field
across the liquid crystal formulation.
The filled device was treated by the application of 800 Hz 10
V/.mu.m square wave at a temperature just below the upper limit of
SmC* phase resulting in a uniform alignment. This device was then
characterized at 40.degree. C. and their tilt angles were found to
decrease from 30.5.degree. to 25.degree. when the amount of C10 was
increased illustrating the tilt angle tuning behavior of C10. The
response time was also found to decrease from 200 to 50 .mu.s when
the amount of C10 was increased from 17 mole percent to 25 mole
percent.
EXAMPLE 3
C27 and C26 were synthesized, where C27 is a racemized homologue of
C26. The Ps of each of these compounds was measured as tabulated in
the table below.
TABLE-US-00011 Compounds Ps (nC/cm.sup.2) C27 ~4 C26 134
The partial racemic (C27) and chiral (C26) compounds were blended
in 2:1 molar ratio to make Formulation 37. The Ps of this
formulation was found to be 29 nC/cm.sup.2, demonstrating the
ability to tune the Ps of the formulation by controlling the
enantiomeric excess.
EXAMPLE 4
C1 was mixed at a various ratios with C10 which has an
I.fwdarw.SmA.fwdarw.Cr phase sequence. C10 possesses a strong
longitudinal dipole due to the cyano-biphenyl structure, unlike C1
where transverse dipole behavior is exhibited leading to
ferroelectric switching. Formulations 7-9 containing different
amounts of C10 were prepared and their tilt angles were measured at
40.degree. C. Although C10 only exhibits SmA phase, all
formulations exhibited SmC* phase. As shown in FIG. 2, the tilt
angles can be tuned by controlling the composition. In alternative
formulations, e.g., Formulation 13, and Formulation 2, such
additives can be used to tune the tilt angle without the
introduction of a discrete SmA* phase in the formulation.
EXAMPLE 5
C1 was mixed at a various ratios with a commercial formulation C15
which exhibits the conventional ferroelectric phase sequence
I.fwdarw.N.fwdarw.SmA*.fwdarw.SmC*. As shown in the table below,
the phase sequence of the formulation shifts from
I.fwdarw.SmA*.fwdarw.SmC* to I.fwdarw.SmC* as the amount of C15
decreases.
TABLE-US-00012 C1:C15 (weight ratio) Phase Sequence 0:100
I.fwdarw.N.fwdarw.SmA*.fwdarw.SmC* 50:50 I.fwdarw.SmA*.fwdarw.SmC*
62.5:37.5 I.fwdarw.SmA*.fwdarw.SmC* 75:25 I.fwdarw.SmC* 87.5:12.5
I.fwdarw.SmC* 100:0 I.fwdarw.SmC* (neat C1)
As illustrated in FIG. 3, a weaker temperature dependence of tilt
angle is observed in formulations with I.fwdarw.SmC* phase
transitions where the content of C15 is lower. A SmA phase was
introduced as the amount of C15 increased and at the same time, the
temperature dependence of tilt angle also increased. These results
indicate the advantage of formulations with I.fwdarw.SmC* phase
sequence over those with I.fwdarw.SmA*.fwdarw.SmC* and furthermore,
those with less SmA forming component in the formulation leading to
FLC formulation with greater temperature stability of tilt
angle.
EXAMPLE 6
An oligosiloxane liquid crystal composition `Formulation 19` was
prepared by mixing the following compounds at the quantities shown
in the table below. The resulting formulation was characterized to
have the phase sequence as shown in Table 6 with SmC* range
spanning between -29 and 50.degree. C.
TABLE-US-00013 Formulation 19 Molar Composition C17 85 C23 15
A transmissive liquid crystal device was prepared by filling a cell
with Formulation 19 as described in Example 2. Treatment of the
filled device by the application of a 30 Hz 10 V/.mu.m square wave
while being held at ambient temperature resulted in formation of
uniform alignment with a contrast ratio of 9:1. A commercial
organic ferroelectric liquid crystal formula from AZ Electronic
Materials (Clariant) Felix 015/000 (`Compound` 15) had a contrast
ratio of 26:1 under the same conditions. The device prepared using
formulation 19 was found to show voltage-on to 90% transmission
response time of 64 .mu.s and 135 .mu.s, Ps of 11 nC/cm.sup.2 and
16 nC/cm.sup.2 and tilt angle of 24.degree. and 27.degree., at
25.degree. C. and 40.degree. C., respectively. Good bistability
with >90% signal retained 20 ms after application of 10 V/.mu.m
200 .mu.s pulse at 25.degree. C. (FIG. 4a). The device prepared
using formulation 19 also showed good bistability at 40.degree. C.
in a cell with 160 nm thick polyimide alignment layer (FIG. 4b) and
driving condition of 130 .mu.s wide 10 V/.mu.m bipolar pulses with
17 ms delay between pulses.
EXAMPLE 7
An oligosiloxane liquid crystal composition `Formulation 23` was
prepared by mixing the following compounds at the composition shown
in the table below. The resulting formulation was characterized to
have the phase sequence as shown in Table 6 with SmC* range
spanning between 39 and 58.degree. C.
TABLE-US-00014 Formulation 23 Molar Composition C19 85 C23 15
A transmissive liquid crystal device was prepared by filling a cell
with Formulation 23 as described in Example 2. Treatment of the
filled device by the application of a 5 kHz, 15 V/.mu.m square wave
while being held at 50.degree. C. resulted in formation of uniform
alignment with a high contrast ratio of 50:1. This device was then
characterized at 25.degree. C. and was found to show voltage-on to
90% transmission response time of 75 .mu.s, a Ps of 24 nC/cm.sup.2
and a tilt angle of 26.5.degree.. The tilt angle was found to show
excellent temperature independence (FIG. 5a). Excellent bistability
was observed when driven by 200 .mu.s wide 10 V/.mu.m bipolar
pulses and with a 20 ms delay between pulses (FIG. 5b).
EXAMPLE 8
An oligosiloxane liquid crystal composition `Formulation 33` was
prepared by mixing the following compounds at the composition shown
in the table below. The resulting formulation was characterized to
have the phase sequence as shown in Table 8 with SmC* range
spanning between -35 and 55.degree. C.
TABLE-US-00015 Formulation 33 Molar composition C17 54.75 C28 20.25
C23 25
A transmissive liquid crystal device was prepared by filling a cell
with Formulation 33 as described in Example 2. Treatment of the
filled device by the application of a 30 Hz, 18 V/.mu.m square wave
while being held at ambient temperature resulted in formation of
uniform alignment. This device was then characterized at 25.degree.
C. and was found to show voltage-on to 90% transmission response
time of 132 .mu.s, a Ps of 43 nC/cm.sup.2 and tilt angle of
39.degree. at 10 V/.mu.m. Good bistability was observed when driven
at 500 .mu.s wide 10 V/.mu.m bipolar pulses with 50 ms delay
between pulses (FIG. 6).
The contrast ratio was found to show a relatively low value of 3:1
due to the high value of the tilt angle. The device was also cooled
to sub-SmC* phase where no switching takes place, then reheated to
SmC* phase where the contrast ratio was measured to be 4:1, thus
showing lack of destruction of SmC* alignment.
EXAMPLE 9
An oligosiloxane liquid crystal composition `Formulation 25` was
prepared by mixing the following compounds at the composition shown
in the table below. The resulting formulation was characterized to
have the phase sequence as shown in Table 6 with SmC* range
spanning between 41 and 61.degree. C.
TABLE-US-00016 Formulation 25 Molar Composition C21 85 C23 15
A transmissive liquid crystal device was prepared by filling a cell
with Formulation 25 as described in Example 2. Treatment of the
filled device by the application of a 500 Hz, 18 V/.mu.m square
wave while being held at 50.degree. C. resulted in formation of
uniform alignment with a high contrast ratio of 50:1. This device
was then characterized at 25.degree. C. and was found to show
voltage-on to 90% transmission response time of 200 .mu.s, Ps of 17
nC/cm.sup.2 and a tilt angle of 30.degree.. The tilt angle was
found to show excellent temperature independence (FIG. 7). This
example showed achievement of high contrast ratio in a formulation
despite its tilt angle being beyond the optimal range (i.e.,
contrast ratio should suffer as a result of inferior bright state
due to wide tilt angle). The tilt angle can be adjusted to optimal
range by using the techniques demonstrated in Examples 1 and 2 to
achieve proper property set based on this formulation.
EXAMPLE 10
An oligosiloxane liquid crystal composition `Formulation 20` was
prepared by mixing the following compounds at the composition shown
in the table below. The resulting formulation was characterized to
have the phase sequence as shown in Table 6 with SmC* range
spanning between -27 and 49.degree. C.
TABLE-US-00017 Formulation 20 Molar Composition C17 75 C23 25
A transmissive liquid crystal device was prepared by filling
Formulation 20 into a cell as described in Example 2 with polyimide
alignment layer. Treatment of the filled device by the application
of a 30 Hz, 10 V/.mu.m square wave while being held at ambient
temperature resulted in formation of uniform alignment with a
contrast ratio of 34:1. The alignment was found to be retained
reasonably well after cooling to a phase below SmC*, and the
contrast ratio was found to be 29:1 after reheating.
This device was found to show voltage-on to 90% transmission
response time of 66 .mu.s, Ps of 30 nC/cm.sup.2, and tilt angle of
30.5.degree. at 25.degree. C., respectively. Excellent bistability
was observed at 25.degree. C. (FIG. 8) when the device was driven
by a 133 .mu.s wide, 10 V/.mu.m bipolar pulses with 13 ms delay
between pulses. This example showed achievement of fast response
time in a formulation although the tilt angle beyond the optimal
range. The tilt angle can be adjusted to optimal range by using the
technique demonstrated in Example 2 to achieve proper property set
based on this formulation.
EXAMPLE 11
An oligosiloxane liquid crystal composition `Formulation 31` was
prepared by mixing the following compounds at the composition shown
in the table below. The resulting formulation was characterized to
have the following phase sequence as shown in Table 7 with SmC*
range spanning between 13 and 60.degree. C.
TABLE-US-00018 Formulation 31 Molar Composition C22 50 C11 50
A transmissive liquid crystal device was prepared by filling a cell
with Formulation 31 as described in Example 2 with a polyimide
alignment layer. Treatment of the filled device by the application
of a 60 Hz 20 V/.mu.m square wave while being held at 55.degree. C.
resulted in formation of uniform alignment within 30 min. This
device was then characterized at 40.degree. C. and was found to
show voltage-on to 90% transmission response time of 300 .mu.s, Ps
of 40 nC/cm.sup.2, and tilt angle of 44.5.degree.. The tilt angle
was found to show excellent temperature independence (FIG. 9).
While certain representative embodiments and details have been
shown for purposes of illustrating the invention, it will be
apparent to those skilled in the art that various changes may be
made without departing from the scope of the invention, which is
defined in the appended claims.
* * * * *
References